This application claims the benefit of Korean Application No. 00-520 and Korean Patent Application No. 00-521, both filed Jan. 6, 2000, in the Korean Patent Office, the disclosures of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to an optical pickup capable of effectively correcting aberration caused by the tilt of an optical disk, with improved defocus properties, in which two light spots, a main light spot and an auxiliary light spot having optical aberration, are focused on the optical disk, and then received by a photodetector and analyzed for the correction.
2. Description of the Related Art
In general, optical pickups are utilized in recording information on or reproducing information from an optical disk. As the recording density of optical disks increases, the need for a light source emitting short wavelength light, and an objective lens having a higher numerical aperture NA increases. On the other hand, when information is recorded on or reproduced from an optical disk using an optical pickup, if the optical disk is arranged at a predetermined angle with respect to the optical axis, i.e., if the recording surface of the optical disk is tilted with respect to the optical axis, coma aberration occurs due to the tilting of the disk. As for an optical pickup for high-density recording, which adopts a light source emitting short wavelength light and an objective lens having a higher NA, when a high-density optical disk is adopted, spherical aberration is caused in an optical system that is compatible with a low-density optical disk, by variations in the thickness of optical disks and the wavelength of light. Furthermore, as the NA of the objective lens increases, the focal depth decreases to control reproduction properties. As a result, there is a need for accurately controlling defocusing caused by an error in adjusting the working distance between the optical disk and the objective lens.
Coma aberration W31 satisfies the following condition (1). Coma aberration W31 suddenly increases at a higher NA, compared to a lower NA, if the tilt angle of the optical disk is the same.
W31xe2x88x9dNA3xe2x80x83xe2x80x83(1)
Recording density of an optical disk is determined by the wavelength xcex of light emitted from a light source, and the NA of an objective lens, which is expressed by formula (2):                               diameter          ⁢                      xe2x80x83                    ⁢          of          ⁢                      xe2x80x83                    ⁢          light          ⁢                      xe2x80x83                    ⁢          spot                ⁢                  xe2x80x83                ∝                  λ          NA                                    (        2        )            
Spherical aberration W40d caused by a thickness variation xcex94D of optical disks satisfies the following formula (3):                               W                      40            ⁢            d                          =                                                            n                2                            -              1                                      8              ⁢                              n                3                                              ⁢                                    (              NA              )                        4                    ⁢                      xe2x80x83                    ⁢                                    Δ              ⁢                              xe2x80x83                            ⁢              d                        λ                                              (        3        )            
where n is the refractive index of an optical disk substrate, and d is the thickness of the optical disk substrate.
In order to increase the recording density of the optical disk to about 15 gigabytes (GB) per disk or more, based on formula (2), a light source capable of emitting short wavelength light of about 410 nm, and an objective lens having an NA of 0.6 or more are needed. However, when the NA of the objective lens is increased for high-density recording, spherical aberration W40d caused by thickness variations of optical disks, which is proportional to the fourth power of the NA, highly increases.
Referring to FIG. 1, a conventional aberration correcting apparatus, which is designed to correct both coma aberration and spherical aberration, includes an objective lens 3 for focusing incident light, and a condensing lens 5 for focusing more incident light to form a light spot on an optical disk 1.
In the case where the optical disk 1 is tilted in a predetermined direction, the condensing lens 5 is driven to be tilted parallel to that direction, thereby correcting coma aberration.
In the conventional aberration correcting apparatus having the configuration above, the objective lens 3 and the condensing lens 5 are driven at a predetermined angle for tracking and focusing control, or only the condensing lens 5 is driven at a predetermined angle, so that the configuration of an actuator for the apparatus becomes complicated.
In addition, the need for high-density recording at 15 GB or more causes the problem of crosstalk to occur due to interference from neighboring tracks when information is reproduced from a track.
To solve the above problems, it is an object of the present invention to provide an optical pickup capable of improving the properties of a reproduction signal, in which coma aberration caused by the tilt of an optical disk and defocus properties can be corrected using at least two light spots focused on the optical disk.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and, in part, will be obvious from the description, or may be learned by practice of the invention.
The above and other objects of the present invention are achieved by an optical pickup comprising: a light source emitting light; light splitting means for splitting an incident beam into at least two light beams including a first beam and a second beam so as to form at least two light spots on an optical disk; optical path changing means for changing the traveling path of the first and second beams reflected from the optical disk; an objective lens focusing the first and second beams to form a main light spot and an auxiliary light spot, respectively, on the optical disk; a photodetector having first and second light receiving portions receiving the first and second beams reflecting from the optical disk and passing through the optical path changing means, respectively, and performing photoelectric conversion on the received beams to produce and detect electrical signals; and a signal processor correcting aberration caused by tilting of the optical disk, using the electrical signals.
In another embodiment, there is provided an optical pickup comprising: a light source emitting light; light splitting means for splitting an incident beam into at least two light beams including a first beam and a second beam so as to form at least two light spots on an optical disk, wherein one of the first and second beams includes optical aberration; optical path changing means for changing the traveling path of the first and second beams reflected from the optical disk; an objective lens for focusing the first and second beams to form a main light spot and an auxiliary light spot, respectively, on the optical disk; a photodetector having first and second light receiving portions receiving the first and second beams reflecting from the optical disk and passing through the optical path changing means, respectively, the first and second light receiving portions each including a plurality of light receiving areas receiving and photoelectrically converting central light and peripheral light, separately, of the first and second beams to detect electrical signals; and a signal processor correcting crosstalk, and aberration caused by tilting and thickness variation of the optical disk.